Recently, as electronic devices have become smaller, there is a need for a high-capacity secondary battery, and particularly, lithium secondary batteries having higher energy densities than nickel cadmium batteries and nickel hydrogen batteries have drawn attention.
As a cathode active material of the lithium secondary battery, a lithium-containing cobalt oxide (LiCoO2) has been mainly used, and in addition to the material, the use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and a lithium-containing nickel oxide LiNiO2 has also been considered. Among the cathode active materials, LiCoO2 has been most widely used due to excellent life characteristics and charge and discharge efficiency, but has limitations in cost competitiveness to be used in a large amount as a power source in the medium and large battery field such as electric vehicles because LiCoO2 has a small capacity and is expensive due to resource limitations of cobalt used as a raw material. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages in that the manganese resources used as a raw material are abundant and thus inexpensive, environmentally friendly, and excellent in thermal stability, but have problems in that the capacity is low, and high temperature characteristics and cycle characteristics, and the like are poor.
In order to supplement these disadvantages, the demand for the Ni rich system as a cathode active material of a secondary battery began to increase. The active material of the Ni rich system has an excellent advantage in that high capacity is exhibited, while there occurs a deterioration phenomenon of the battery performance due to a side reaction with an electrolyte solution and lithium-containing impurities remaining on a surface of a cathode material. In particular, with respect to the cathode active material with a layered structure, which has a high capacity, due to irreversible reactions occurring from a high content of the remaining lithium, the coulomb efficiency becomes low, a decrease in cycle life occurs, and an aspect that the above-described deterioration in performance of the battery becomes worse as the potential becomes high is shown.
A technology for suppressing the above-described lithium-containing impurities has been currently developed, but a satisfactory result has not yet been elicited. Therefore, there is a desperate need for developing a cathode active material having a novel configuration, which may enhance the electrochemical properties and reliability of a lithium secondary battery for a long period of time.